The present invention broadly relates to electrical circuitry which maintains the power in the output lines of such circuitry below a predetermined level in order to prevent the possibility of explosion if these output lines or any circuit electrically connected therewith comes in contact with a potentially explosive atmosphere.
Intrinsically safe apparatus or circuits are ones in which any spark or thermal effect produced either normally or under prescribed test conditions is incapable of causing ignition of a flammable or combustible material in air in its most easily ignitible concentration. Thus, the definition of "intrinsically safe" must include all circumstances which can reasonably be expected to occur in use of the apparatus or circuit and must include the conditions which arise when internal faults occur in the system. There are a number of independent or governmental testing or certifying organizations such as Underwriters Laboratories, Inc., Factor Material Engineering and Canadian Standards Association which provide prescribed test conditions for determining if equipment is intrinsically safe.
In the design of intrinsically safe equipment, there will typically be electrical circuitry enclosed in a "safe" environment (e.g. explosion proof housings) and those circuits which extend from this safe environment into a potentially dangerous area where the circuit may come in contact with an explosive atmosphere. These circuits which extend from a safe environment to a potentially dangerous area are typically designed so that they pass through a safety barrier which is designed to limit the power which can enter the circuit. Accordingly, as used herein, the term "intrinsic barrier" is defined as a device which will not permit the transfer of more than a precisely restricted amount of energy from one side of the barrier to the other.
FIG. 1 is one of a series of charts which are published by one of the above mentioned testing organizations. These charts present the minimum igniting currents for specific gases and the testing organization will typically specify that the maximum permissible working current can be a certain percentage of the values shown and thereby resulting in a safety factor. As seen in FIG. 1 the minimum igniting current will depend upon the gas or explosive environment involved. For example, circuit currents falling below the hydrogen curve in area "A" would be considered safe for use in an air-hydrogen environment as well as being safe in air mixed with any of the three other gases shown. However, circuit currents falling between the hydrogen and ethylene curves (i.e. in area B) would be considered safe for an ethylene-air environment but not safe for use in hydrogen and air. Furthermore, it will be seen that the lower the voltage, the greater the current that will be permitted in the circuit without exceeding the minimum safe level. This relationship is important since there is increasing use sophisticated electronic components and microprocessors have relatively high current requirements, and to have these components operate in, for example, the environments listed in FIG. 1, it becomes necessary to design intrinsic barriers which will greatly limit the voltage so as to thereby permit the use of larger values of currents.
As a result of this voltage current relationship, designers will generally attempt to reduce the circuit voltage (e.g. to the 15-100 V range in FIG. 1.) in order to facilitate the safe use of relatively higher levels of current to operate the electronic circuitry contemplated. Thus, the design of intrinsic barriers requires the selection and arrangement of individual components to provide the required level of current in the output lines and, at the same time, insure that the current will not surge above a predetermined level in the event of failure of any components in the barrier.
Intrinsic barriers are also typically encapsulated in order to prevent tampering and it is desirable in these encapsulated designs to have means for non-destructively testing selected components in the barrier.
Electronic equipment also frequently requires the use of multiple barriers to protect different circuits and it is highly desirable to provide these barriers stacked together and encapsulated into a single intrinsic barrier. It is also desirable if these individual barriers can be designed to control different levels of voltage.
Prior art barriers have typically included a series of resistors and resistor/zener diode combinations or Redding barriers. However, these essentially passive devices have been found unsuitable for the operation of some electronic and electromechanical equipment. Accordingly, there is a continuing need for an improved intrinsic barrier having combination of active and passive electrical components which facilitates the safe utilization of electronic components in explosive environments. Likewise, there is a need for active intrinsic barriers in producing a source of controlled voltage. As also noted above, there is also a continuing need for intrinsic barriers that are designed to permit the individual testing of the elements in the barrier. Finally, there is a need to provide multiple barriers combined into a single intrinsic barrier and with the individual barriers being capable of controlling different levels of voltage.